This invention relates to a nonwoven fabric and more particularly to a nonwoven fabric presenting no significant strength difference between in its machine direction (MD) and in its direction (CD) intersecting the machine direction and provided on its surface with a plurality of irregularities. This invention relates also to a process for making such a nonwoven fabric.
Japanese Patent Application Disclosure No. 1996-60509 describes a process for making a nonwoven fabric used as a wiper. According to this known process, a laminate consisting of a hydrophilic web and a hydrophobic web is continuously fed in one direction and the laminate is, as it is fed, subjected to high pressure water streams injected from fine orifice nozzles. In this manner, component fibers of the hydrophilic web and the hydrophobic web are rearranged and mechanically intertwined together to form a nonwoven fabric. This nonwoven fabric is then subjected to a heat treatment in order to crimp composite fibers contained in the web and thereby to form a plurality of irregularities on the surface of the nonwoven fabric. When this nonwoven fabric is used as a wiper, these irregularities formed in this manner function to scratch off dust and dirt from an object which should be cleaned.
As the nonwoven fabric obtained by subjecting the laminated web continuously fed in one direction to the high pressure columnar water streams, many of the component fibers lying immediately below the columnar water streams are forcibly moved aside. As a result, trough-like regions are defined by the component fibers of its surface density reduced by the moved aside fibers and crest-like regions are defined by the component fibers of its surface density increased by accumulated fibers. These trough- and crest-like regions extend in the web feed direction and are alternately arranged transversely of the web. Such a process is well known to those skilled in the art. It is also well known that such nonwoven fabric has a relatively high tensile strength and a relatively low stretchability in the MD but a relatively low tensile strength and a relatively high stretchability in the CD. In other words, the tensile strength as well as the stretchability of this nonwoven fabric largely depend on the direction. Accordingly, when it is desired to use this nonwoven fabric as the wiper, its orientation must be considered to avoid a possibility that the nonwoven fabric might be easily torn.
An object of this invention is to provide a nonwoven fabric requiring no consideration of its orientation to ensure the desired tensile strength as well as the desired stretchability and a process for making such a nonwoven fabric.
This invention aiming to achieve the object set forth above comprises a first invention relating to the nonwoven fabric and a second invention relating to the process for making this nonwoven fabric.
According to this first invention, there is provided a nonwoven fabric having first and second surfaces extending in parallel to each other and formed by component fibers mechanically entangled between the first and second surfaces, regions of high fibers surface density and regions of low fibers surface density continuously or intermittently extend in parallel one to another in one direction so as to be alternately arranged in a direction transversely of the one direction and to form undulations defined by crests corresponding to the regions of high fibers surface density and troughs corresponding to the regions of low fibers surface density; and
the nonwoven fabric further including bridge-like regions each extending transversely of the one direction between each pair of adjacent the region of high fibers surface density and the region of low fibers surface density and having a density higher than that in the region of low fibers surface density.
According to one embodiment of this first invention, the region of low fibers surface density has a width of 0.05-0.5 mm as measured transversely of the one direction.
According to another embodiment of this first invention, the component fibers partially extend transversely of the one direction.
According to still another embodiment of this first invention, the component fibers are short fibers or continuous fibers.
According to this second invention, there is provided a process for making a nonwoven fabric comprising the steps as follow:
a. a step of continuously feeding a fibrous web in one direction;
b. a step of feeding the fibrous web to a region lying immediately below high pressure columnar water streams injected from a plurality of nozzles arranged on a line extending transversely of the web at a range of feed rate deceleration of 16-35% as defined by a formula:
Feed velocity deceleration (%)={(Feed velocity in front of columnar water streams)xe2x88x92(Take-out velocity behind columnar water streams)}/(Take-out velocity behind columnar water streams)xc3x97100; and
c. a step of taking-out the web having been treated by the columnar water streams in the one direction at the range of feed velocity deceleration.
According to one embodiment of this second invention, the columnar water streams are obliquely injected toward the web in the direction opposed to the web feed direction at an angle of 4xcx9c15xc2x0 with respect to a perpendicular line to the web.
According to another embodiment of this second invention, the fibrous web comprises short fibers or continuous fibers.